This invention relates to methods for investigation of an earth formation in the vicinity of a borehole traversing the formation, using measurements obtained by means of a borehole logging operation.
The exploration for a production of subsurface reserves of raw materials such as hydrocarbons involves a variety of techniques for finding possible locations for such reserves and for detecting and assessing them. Thus seismic techniques may be used for initial survey work and borehole logging techniques for detection. Seismic data, obtained by energizing a source and recording sound waves propagating from the source to a multiplicity of detectors, is typically processed to generate an image (or seismic section) of a cross-section of the formation being investigated. A seismic image can be reviewed to identify formation structures likely to harbour hydrocarbons and thus determine suitable locations for drilling of exploration boreholes. One technique for producing such an image is described in a paper `Migration Stack aids interpretation`, by D. W. Rockwell, Oil and Gas Journal, 1971, vol. 69, pp. 202-218, and in U.S. Pat. No. 3,353,151. However, this technique is described solely in the context of analysis of relatively low-frequency seismic signals in circumstances free of noise arising from sonic energy propagation through borehole fluids, and in the context of a relatively simple geometry in which the formation being investigated extends solely to one side of the line connecting the source and detectors.
After a borehole has been drilled certain borehole logging tools, such as the multiple-transducer device described in U.S. Pat. No. 4,468,623, can be used to obtain measurement signals from within a borehole which can be processed to generate images representative of formation structures intersected by the borehole. However, seismic techniques tend to yield relatively low resolution images of large formation sections, whereas the borehole images provide relatively very high resolution images restricted solely to the borehole wall.
There remains a need for images which extend into the formation from the borehole and provide information on the structures in the vicinity of the borehole with at least moderate resolution, irrespective of whether the structures actually intersect the borehole. Such images would be a valuable complement to conventional seismic images of such structures as salt domes. They could yield information on fractures in the vicinity of the borehole but not actually intersecting it, which would be of great value in planning completion and production operations for extraction of hydrocarbons. Likewise the position of an oil-gas interface in a reservoir could be monitored. These images would also assist drilling of relief boreholes to help stop a blow-out of a nearby borehole.
Recently interest has been growing in relation to the possibility of drilling of high-angle or even horizontal extended-reach holes out from a conventional borehole into horizontal oil-bearing strata intersected by the borehole. Such a technique creates a need for accurate information on the position of the drainage well relative to the upper and lower boundaries of the oil-bearing stratum (typically a few tens of meters thick). During production from such a drainage well it may also be necessary to monitor the position of any gas which may be expanding in volume as oil is withdrawn and thus approaching the well. Moderate resolution images of the vicinity of the borehole and the drainage well would clearly be of great utility in these circumstances.
It is an object of this invention to provide a method, using measurements obtained in a borehole traversing an earth formation, for detection and imaging of features of the formation in the vicinity of the borehole, such as fractures, other boreholes and boundaries between different constituents of the formation.